I know that we're all still feeling our way around this issue, but how are other profs handling it? What is good evidence of unauthorized AI use? How do you handle a student who refuses to engage in attempts to get their side of the story?

For my classes, we talk once a month or so about acceptable use (treat it like a not-very-bright friend who's overconfident and prone to hallucinations). It's okay to brainstorm, bounce ideas, and generally use AI to spark creative problem solving. It's not okay to have it do your assignments.